The present invention relates to a device for rapid assessment of coagulation rates. In the preferred embodiment, the clotting ability of small samples of whole blood, or plasma is assessed on the basis of measuremets such as prothrombin time, partial thromboplastin time or similar tests.
The natural phenonmenon of blood coagulation is complex, involving a sequence of enzymatic and physical interactions to convert fluid blood into an adhesive mass. The major processes involve the conversion of the proenzyme, prothrombin, to the enzyme, thrombin and the action of thrombin upon fibrinogen to form fibrin. The fibrin separates as long fibers or threads, which are extremely adhesive. These threads stick to each other, blood cells, tissues, and foreign substances to form a three dimensional network or clot. The adhesiveness causes the clotted blood to hold together and stick firmly to injured tissues to prevent hemorrhage. The overall clotting sequence can be represented as:
Favored by Vitamin K; Inhibited by oral anticoagulants Liver Prothrombin Activated by Ca.sup.+.sup.+ and Thromboplastic substances Thrombin Fibrinogen Fibrin (Clot)
Blood does not normally clot in the vascular system, since thrombin is present in the inactive form, prothrombin, which becomes activated to thrombin when blood escapes due to injury or is withdrawn from the blood vessels. Prothrombin activation is accomplished by substances known as thromboplastins which occur in blood platelets and various tissues, particularly lung and brain. In some diseases, partial prothrombin activation can occur in the blood vessels and result in a thromboembolic condition. This condition is hazardous in diseases such as the following, for which treatment with various anticoagulants is frequently prescribed to prevent the formation of intravascular thrombi and maintain normal hemostatis:
1. MYOCARDIAL INFARCTION
2. RHEUMATIC HEART DISEASE
3. CEREBROVASCULAR DISEASE
4. VENOUS THROMBOSIS
5. PULMONARY EMBOLISM
The administration of anticoagulants as part of the treatment for these diseases is largely empirical and dosage is regulated on a daily basis until the response of an individual patient to the anticoagulant becomes established. Even after regulation of the dosage for long-term treatment, patient response is generally monitored on a bi-weekly or monthly schedule.
The oldest and most widely used method for monitoring patient response to anticoagulant therapy is the one-stage prothrombin time proposed by Quick, or some adaptation of this basic method. In the one-stage prothrombin time measurement, blood is collected in sodium citrate or oxalate, which chelate calcium and prevent prothrombin activation prior to the start of the measurement. The blood is centrifuged and an aliquot of plasma mixed with an excess of thromboplastic extract of brain or lung tissue which contains sufficient calcium to overcome the effect of the chelating agent. Mixing is carried out rapidly under controlled conditions and the time required for incipient clot formation defined as the prothrombin time. Clot formation may be observed visually or measured by means of commercially available mechanical devices. When the measurement is carried out at 37.degree.C, normal human plasma usually has a prothrombin time of 12 seconds.
In current clinical practice, measurements of prothrombin time are made on blood samples drawn from patients and then transported to a laboratory for analysis. Though these measurements are relatively easy to perform and do not involve the use of complex or expensive reagents, they do require the time of specifically trained personnel, along with the use of unique equipment and laboratory facilities. The need for a laboratory analysis introduces a significant delay between the time a sample is drawn and that at which the analytical result is available to aid in the regulation of thereapy. While the delay is not usually of a critical nature, it represents an inconvenience to both the patient and prescribing physician. A major shortcoming of the current methodology is the need for repeated venipuntures over a long period of time to obtain the blood samples necessary for monitoring anticoagulant therapy for each patient after a regimen has been established. Since the patients concerned are frequently elderly, these venipunctures can be difficult to carry out and involve some trauma to the patient.
The present inventon describes a device which may be applied to rapid measurements of prothrombin time on a drop of uncoagulated whole blood obtained from a finger puncture. The device includes all of the reagents and apparatus necessary for carrying out a prothrombin time measurement. The device thus represents a significant improvement over the prior art. By means of the device, measurements of prothrombin time can be carried out in approximately 2 minutes to produce a definitive result, eliminating delays and the need for trained personnel with access to specific laboratory facilities. The sample required for each measurement is a drop of uncoagulated whole blood from a finger puncture, eliminating the inconvenience of venipuncture necessary in the methodology of the prior art. In addition, there is no critical time for reading the result. The test may be started and the result veiwed and recorded at the convenience of the user. The final position of the result endures nearly indefinitely.
Prothrombin times are usually measured in small containers such as test tubes or cells into which a plasma sample is placed together with reagents such as one of various commercial thromboplastin reagents together with extra calcium, if required. The time from mixing of the reagents and the sample to clot formation may be measured by various procedures including simple observation by eye, by optical instrumentation which detects changes in transmission of the sample, by fibrometers which detect the adhesion of the clots to wires or fibers, by measurement of conductivity changes and by various measurements of viscosity increases. The initial one stage prothrombin time was developed, demonstrated and popularized by Quick. This test is sensitive to blood Factors V (the Labile Factor or Proaccelerin), VII (Proconvertin) and X (Stuart-Power Factor) as well as Prothrombin (Factor II). In determination of the prothrombin time by the Quick method the factors with the exception of prothrombin are assumed present in excess. However, Factors VII and X may decrease during anticoagulant therapy.